I have two domains that are both hosted on the same server. Therefore, they both have the same index.html page, and they share all of the other pages. This means that there are two ways to access every file stored on the server:
domain1/file
And
domain2/file
Is there a way to redirect the user to the respective domain1 URL whenever they go to a domain2 URL? The catch is that I only want to redirect if a domain2 URL is gone to.
How can I achieve this programmatically?
Just because you have two domains running on one server does not mean they have to share index.html. The way around this is by using Virtual Hosts. You didn't mention which web server type you are using, so I'll give you an apache example:
<VirtualHost *:80>
DocumentRoot /www/example1
ServerName www.example.com
# Other directives here
</VirtualHost>
<VirtualHost *:80>
DocumentRoot /www/example2
ServerName www.example.org
# Other directives here
</VirtualHost>
This allows you to have two directories, each serving as a root path for each domain. You'd put the domain1 files in /www/example1, and the domain2 files in /www/example2, in this example. There are some other configuration options you may need, but again depending on your setup, they could vary greatly.
If you are using IIS, there's a writeup over on Server Fault that has information on how to perform that. (This question probably belongs there anyway).